Method of and means for neutralizing inductive disturbances in magnetic reproducers



M. CAMRAS 2,351,008 METHOD OF AND MEANS FOR NEUTRALIZING INDUCTIVE June 13, 1944.

DISTURBANCES IN MAGNETIC REPRODUCERS Filed Nov. 18, 1942 24 29 3/ #M M fig Patented June 13, 1944 METHOD OF AND MEANS FOR NEUTBALIZ- ING INDUCTIVE DISTURBANCES IN MAG- NETIC REPRODUCEBS Marvin Cami-as, Chicago, 111., assignor to Armour Research Foundation, Chicago, 111., a corporation of Illinois Application November 18, 1942, Serial No. 465,961

Claim.

This invention relates to improvements in a method 0! and means for neutralizing inductive disturbances in a magnetic reproducing or magnetic recording device so that only the inductive impulses created by a magnetized recording medium will be amplified and reproduced, while inductive disturbances resulting from stray magnetic fields in the vicinity of the reproducing element or initial pick-up element will be neutralized and balanced out, whereby fidelity of reproduction is maintained.

The invention is highly desirable for use in connection with a magnetic recording and reproducing device wherein sound is recorded by magnetizing an elongated recordin medium such as a paramagnetic tape or paramagnetic round wire. Frequently the recording device may also be used as a reproducing device to reproduce what has been magnetically recorded on the elongated medium. -In other cases, one may be used for reproduction only. The instant invention is designed to make that reproduction oi greater fldelity by the neutralizing of inductive disturbances resulting from stray magnetic fields.

In such a magnetic reproducing device, there is usually an amplification circuit, a driving motor, power transformer, and other electrical elements used in the various circuits, such as a choke coil. each of which may provide a stray magnetic field which will also induce a voltage in addition to the voltage induced by the recording medium itself. While such stray fields will theoretically resolve into a single resultant stray field, such resultant stray field is not always 01' the same magnitude nor in the same direction, since other stray fields from various electrical appliances or devices operating in the vicinity of the magnetic reproducer will likewise induce voltages in the pick-up elements, and these other devices may be turned on and oil at indeterminate times, so that the ultimate resultant field in most cases is continuously fluctuating as to both direction and magnitude. Consequently, in a magnetic record player, wherein the input magnet or pickup magnet is designed to respond to minute magnetic variations along a. fine wire or other elongated recording medium, the stray hum pick-up may at times be event greater than the signal voltage from the wire. or course, there are two remedies tor stray hum pick-ups. One is to enclose the magnetic input device or pick-up device in a shield, but this remedy is extremely costly if done efl'ectively. Second, the magnetic pick-up device or input device might be oriented until the hum pick-up is at a minimum but this will not eliminate the bum pick-up from stray magnetic fields completely except under certain special conditions which very rarely occur in practice, namely, a single "poin source 01' alternating current magnetic fields, or several sources which give the fundamental components all in time phase, and harmonic components also in time phase and in the same direction as the fundamental components. With a resultant field that varies continuously as to both direction and magnitude, it is substantially impossible to orient the reproducing magnet to reduce the hum pick-up.

In addition, it may be mentioned that the above two methods are not always practical with certain types of magnetic record playing devices for mechanical and practical reasons. For example, where the same magnet device is used to magnetize a wire or other elongated recording medium, and then later used as the pick-up magnet for reproducing a recording previously made on that medium, it is not feasible to change the position of this magnet device, because the travel of the wire through the machine is along a substantially fixed path.

With the foregoing in mind, it is an important object 01 the present invention to neutralize inductive disturbances resulting from stray magnetic fields in a magnetic reproducer having a pick-up magnet located in a fixed position. and without eflectlvely shielding the magnet.

Another object of the invention resides in the neutralization of inductive disturbances resulting irom stray magnetic fields in a magnetic reproducer by the introduction of an auxiliary coil in series with the pick-up or input magnet; or, it so desired, the auxiliary coil may be in parallel with the input or pick-up magnet.

Another object of the invention is the elimination of hum pick-up in a magnetic reproducer by the utilization of an auxiliary coil of approximately the same size and construction as the pick-up magnet, and orienting the auxiliary coil relatively to the pick-up magnet so as to balance out induced voltages from stray fields.

A further feature of the invention resides in the provision or an auxiliary coil in the input amplification circuit of a magnetic reproducer to balance out inductive disturbances resulting from stray magnetic fields.

Still another feature of the invention resides in the introduction of an auxiliary pick-up coil in a location in close proximity to the input or pick-up magnet of a magnetic reproducer, for the purpose of balancing out disturbances from stray magnetic fields.

A further ieature of the invention resides in the provision of a hum reducing arrangement including the use of an auxiliary pick-up device in a region where the magnetic field is substantially an image of the field near the maior pickup device in respect to stray field conditions, and orienting the auxiliary pick-up device so as to neutralize the inductive effects of the stray field.

A further object of the invention is the provision of a new and novel method of eliminating, neutralizing, or balancing out the inductive disturbances resulting from stray magnetic fields, in connection with a magnetic reproducing device.

While some of the more salient features, characteristics and advantages of the instant invention have been above pointed out, others will become apparent from the following disclosures, taken in conjunction with the accompanying drawing, in which:

Figure l is a fragmentary front elevational view of a magnetic recording and reproducing device, equipped with means embodying principles of the instant invention and incorporating the practice of my novel method in its use;

Figure 2 is a simplified diagram of the input portion of the amplifying circuit embodied in the device of Figure l, and also including principles of the instant invention; and

Figure 3 is another simplified diagram of the same portion of the amplifying circuit as Figure 2, but illustrating a different connection for the auxiliary pick-up coil.

As shown on the drawing:

The illustrated embodiment of the instant invention is shown associated with a magnetic recording and reproducing device including a cabinet or other supporting structure 5 having a front panel 6 carried thereby. All of the operating equipment of the recording and reproducing device which need be described herein may be carried or supported by the panel 6.

Near one side of the panel a reel or spool I is rotatably mounted, and near the opposite side a similar reel I is rotatably mounted. These reels may be operated in both directions so as to wind a recording medium backward and forward therebetween in either direction. In the present instance, the recording medium is shown in the form of a very fine round wire 9, which may be of approximately the size of a human hair, .004 or .005 inch in diameter being satisfactory.

With the illustrated recording and reproducing device, the recording medium or wire 8 travels from reel I to reel 8 during the making of a recording, or during the magnetization of the wire itself in keeping with variations in the sound of the production being recorded upon the wire. After a recording has been made, the wire may be rewound of! reel l onto reel I, and during this rewinding operation nothing happens to the Wire or the recording thereof. The wire may be then moved in the forward direction, namely from reel I to reel 8, and the recording on the wire reproduced audibly by means of a suitable inductive pick-up and amplifying circuit, utilizing the same magnet as a pick-up magnet for reproduction that initially magnetized the wire during recording. Obviously, other circuits will be involved inside the device for reproduction, and the instant invention centers itself mainly in the reproduction operation of the device. It is not essential that the device be made to both record and reproduce, but such is a convenient form.

As the wire travels from reel I to reel I it assaoos first passes through a level wind element II associated with reel 1, thence through an erasing head I I designed to demagnetiae the wire, around a guide pulley I, through a recording head in a housing ll. around another guide pulley I I, through a level wind arrangement II, and onto reel During a reproduction from a wire previously magnetized. the erasing head H does not function, and the recording head in the casing it acts as the reproducing or pick-up magnet which may for convenience be termed the input magnet of the amplifying circuit. With reference to Figure 2, it will be seen that this input magnet includes a paramagnetic core Ii, preferably laminated, having a relatively minute air gap I! to establish in effect pole pieces of opposite polarity on each side of the air gap. Around one portion of the core I! a winding or coil Ills positioned. It will therefore be seen that during a reproduction, as the wire 9 passes over or through a slot in the pole pieces on each side of the air gap l1. the magnetization on this wire will induce a voltage in the coil II, and this induced voltage will vary in accordance with the varying magnetization of the wire.

Other voltages from stray magnetic fields in the vicinity will also be induced in the coil II, and in view of the fact that this coil is connected in the input portion of the amplifying circuit, it is desirable to neutralize or balance oil the effect of the induced voltage from stray fielc s, and such is efilciently accomplished by the instant invention.

With reference to Figure 2 it will be seen that the coil 18 is connected at one end through a conductor l9 to the grid of an amplifying tube 20 which is diagrammatically illustrated as a triode, whereas in practice a pentode including both screen and suppressor grids would probably be used, since this tube forms the first stage of amplification. A uni-cell battery 2| may be inclined in the conductor ID as a grid bias. The other end of the coil II is connected through a conductor 22 to a coil 23 which in the form of an auxiliary pick-up coil and preferably is approximately the same construction and the same number of turns as the coil iii. In this illustrated instance, the auxiliary pick-up coil 23 is in series with the coil I 8, and the opposite end of the coil :3 is connected through a conductor 24 to a source of energy such as a battery 25. The cathode of the tube 20 may be connected through conductor 26 to the conductor 24. The plate of the tube 20 is connected through conductor 21 to a load resistance 2| which in turn is connected as indicated at 29 to the opposite side of the battery 25. Thus, we have a simplified input portion of an amplifying circuit, including the first stage of amplification, and further stages of amplification may be connected as indicated b extended conductors Ill and ii, a condenser 8 preferably being included in the conductor ll.

In operation, the pick-up coil it will not only have a voltage induced therein by virtue of the magnetized wire 8. but will also have another varying voltage induced by the resultant of stray magnetic fields. These stray magnetic fields may originate in a power transformer, driving motor, choke coil, and similar apparatus used in the magnetic reproducing device itself. Likewise. there may be additional stray fields from electrical apparatus operated in the vicinity of the magnetic reproducing device. If the resultant of all these stray fields was constant, the pick-up or input magnet still could not be oriented efiectively because by virtue of the travel of the wire I. this magnet must remain substantially in fixed position.

The auxiliary pick-up coil is therefore used. and this coil is preferably placed as closely as possible to the coil II so as to be in a region oi stray magnetic fields which is an image of the region in which the coil I8 is located. As seen in Figure l, the coil 23 is positioned as closely as possible to the coil II, this being the preierable location. With such positioning, any voltage induced by a stray field in the coil is will likewise be induced by the same field in the coil 23, and the resultant stray field will have the same efiect on the coil 23 as it does on the coil i8, and will be oiJhe same magnitude and the same direction as to both coils.

The coil 23 is oriented to cancel or balance out the voltages induced from stray fields. and this orientation may be by opposite connection. opposits winding. or in some other suitable manner. In the illustrated instance, the coil 13 is oppositely wound to the coil II. and therefore the voltages induced by stray fields in the coil 23 will be in opposition to the voltages induced by the same stray fields in the coil II, and thus there is a balancing out 01' all inductive disturbanoes resulting from stray fields. The voltage induced in the coil II by the magnetized wire or recording medium will not be induced in the coil 23 by virtue of its position, so the result will be a true and accurate amplification of the inductive impulses from the magnetized recording medium alone, namely, fidelity of reproduction without any hum or other noise resulting from stray field disturbances.

course. voltages induced in the coil II by the magnetized recording medium will disrupt the bias on the grid of the tube 20, amplification of these impulses will result. and the amplification may be reproduced at audio frequency through a loud speaker. all in a manner known to those skilled in the art.

The coil II does not oi necessity have to be connected in series with the pick-up or input coil II. In Figure 3, I have illustrated substantially the same simplified diagrammatic showing of an input portion oi the amplifier circuit as is illustrated in Figure 2. The same reference numorals have been used to designate like parts, but in this instance the conductor 22 is eliminated. and conductor 24 runs direct from the battery to one side of the pick-up or input magnet coil is. The auxiliary pick-up coil 23 is shown connected between conductors l0 and Il, in parallel with the coil ll. with this arrangement, it will be seen that the hum balancing or auxiliary pickup coil II is in a region oi stray fields which is the exact image of the region of stray fields occupied by the coil ll. Again. the coil ll is oriented by being wound oppositely to the coil ll, so that voltages induced by stray fields in the coil II will be neutralized by equal and onposite voltages induced by the same stray fields in the coil 28.

My novel method of neutralizing inductive disturbances in a magnetic reproducing device is believed sufiiciently apparent from the foregoing as not to warrant further detailed description herein.

From the foregoing. it is apparent that I have provided novel means and a novel method of neutralizing or balancing out inductive disturbances occurring in the input circuit of a reproducing device. It is apparent that by the aid of an auxiliary coil, properly oriented, and arranged to oppose stray pick-up oi the magnetic input device, inductive disturbances resulting from stray magnetic fields may be balanced out of the magnetic input device, leaving only the eflect oi the intended and proper magnetic field to be amplified. It is further apparent that my novel arrangement is simple in construction. and extremely economical to incorporate in a magnetic reproducing system and to operate.

It will, of course, be understood that various details oi construction may be varied through a wide range without departing from the principles of this invention and it is. therefore. not the purpose to limit the patent granted hereon otherwise than necessitated by the scope of the appended claims.

I claim as my invention:

1. In a device for reproducing magnetic recordings, an input magnet responsive to magnetic variations along an elongated recording medium, said magnet having a coil. and an auxiliary pick-up coil connected in circuit with the magnet coil and arranged to substantially neutralize the efiect oi stray voltages induced in the magnet coil regardless of the general location oi said device.

2. In a magnetic reproducing device, a panel. a pair of spaced reels thereon. an elongated recording medium on said reels, a reproducing head including an electromagnet between said reels in the path or said recording medium. and a pick-up coil on said panel immediately adjacent the coil of said electromas'net in circuit connection therewith and orientable to substantially neutralize the eifects of stray voltages induced in said electromagnet regardless of the general location of said device.

3. The method oi neutralising inductive disturbances in a pick-up magnet tor a reproducer of magnetic recordings. including the step of setting up a reactionary inductance by an induction coil located in a region oi stray magnetic fields which is an image oi the stray magnetic region or said pick-up magnet and orienting said induction coil with respect to said pick-up magnet to balance out the effects oi a particular stray magnetic field region of the pick-up magnet depending upon the location oi the apparatus.

4. In a device for reproducing magnetic recordings, an input magnet responsive to magnetic variations along an elongated recording medium, said magnet having a coil, and a hum bucking coil connected in circuit with and disposed adlacent said magnet coil and orlentable to substantially neutralize the efiects of inductive disturbances in said magnet caused by sources foreign to said device and apparatus connected therewith.

5. The method of neutralizing inductive disturbances in a pick-up magnet for a reproducer oi magnetic recordings caused by stray fields from sources external to the reproducer and its associated apparatus. including the steps oi setting up an opposing inductance by an induction coil in circuit with said pick-up magnet. and orienting said coil to balance out the efiects oi said stray fields.

MARVIN CAMRAS. 

